3rd generation partnership project (3GPP) long term evolution (LTE) is an improved version of a universal mobile telecommunication system (UMTS) and is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) that is an evolution of the 3GPP LTE.
To mitigate interference due to uplink transmission, a transmit power of a user equipment needs to be adjusted. If the transmit power of the user equipment is too low, a base station barely receive uplink data. If the transmit power of the user equipment is too high, the uplink transmission may give too much interference to other user equipments' transmission.
A power headroom reporting procedure is used to provide the serving base station with information about the difference between the nominal maximum transmit power and the estimated power for uplink transmission.
Recently, with the introduction of multiple carriers, one user equipment can receive a service from a plurality of serving cells. With the introduction of the plurality of serving cells, there is a need for a method in which a user equipment calculates power headrooms for the plurality of serving cells and performs power headroom reporting.